For that story, I spoke to three sources: Tumblr CEO David Karp, Tumblr board member and investor Roelof Botha, and an anonymous source close to the company.

Karp, Botha, and my anonymous source were extremely candid about the state of the company.

They told me Tumblr's Q1 revenues were behind plan, that Tumblr was raising money, that Tumblr was holding a board meeting that week, and that it was in the final stages of interviewing candidates for a COO job.

They are not being so candid this time around!

In fact, they are not even replying to my emails or voicemails.

This makes sense: People stop talking when they are scared that talking will keep something good from happening.

So, having spoken to those people so recently and having a decent grasp of Yahoo and Tumblr, what do I think is going on?

I have four thoughts, actually!

A $1 billion sale is not what Tumblr investors were hoping for, but they'll take it.

Tumblr raised $85 million at an $800 million valuation back in 2011.

This sale is not much of a premium over that valuation. But, you have to assume that Tumblr's later stage investors bought "preferred" shares in the company.

The benefit of preferred shares is that they sometimes guarantee a return of 1.5X or 2X in a liquidity event.

So, even though Tumblr would be selling at a 25% increase in valuation, its late-stage investors could be getting something like a 50% return.

In an era following the Facebook IPO, that's not something to sniff at.

Marissa Mayer is perfectly positioned to give Tumblr CEO David Karp what he wants.

You might think David Karp would sell Tumblr to Yahoo for $1 billion because it would put $250 million in his pocket.

You would not be stupid to think this.

$250 million is a nice incentive for anyone, let alone a first-time entrepreneur in his 20s, like Karp.

But…

Karp has gotten lots of offers to sell Tumblr at prices that would make him rich over the years, and he's resisted them all.

Why would he sell this time?

Easy: Karp and Tumblr have problems like never before…

Tumblr does not have a COO and the board wants him to hire one,

Tumblr is light on revenues. (Karp disputed this in an earlier story, but sources told us Tumblr is doing OK on revenues – it's marginally behind plan.)

Tumblr needs cash to keep running and growing.

Selling to Mayer solves all those problems.

To keep Tumblr's servers going, Yahoo has billions of dollars in cash. As for revenues and a COO?

Mayer, we're guessing, will tell Karp not to worry about either for now.

Mayer, according to lots and lots of sources who have worked with her, is not concerned with things like "revenue" and "money." She's much more focused on "product" and "user-growth."

Right now, Yahoo shareholders are happy to let her do that. That's because Yahoo shareholders are not shareholders because of Yahoo's core business or core products.

They are Yahoo shareholders because owning Yahoo is the only way to make a pre-IPO bet on Alibaba, a successful Chinese Internet company that Yahoo owns a big stake in.

So Mayer is perfectly able to spend a billion dollars on Tumblr and allow Karp to keep running it relatively ad-free. This in turn, should keep Tumblr growing, since users hate ads.

Sometime in the next couple years, more people will be accessing the Internet through mobile devices than through PCs.

This trend is bad news for Yahoo, which has limited mobile reach and even more limited mobile usage.

(Its reach numbers get a boost because Yahoo gets credit for having a weather app installed by default on every new iPhone. This is despite the facts that the Weather Channel provides the data for the app, Apple built the app, and Yahoo sells no ads in the app.)

Tumblr, despite a rocky start in mobile, now has TONS of mobile users.

Check out this table from ComScore, which shows that Tumblr is, relatively speaking, about as strong as Facebook in mobile, and much stronger than Yahoo:

ComScore

Facebook-Tumblr doesn't seem like a fit to me.

Facebook bought Instagram for a billion dollars a year ago. Some people are reporting that it is considering paying the same price for Tumblr today.

But the only reason Facebook bought Instagram was that Instagram was a huge disruptive threat to Facebook's business.

Facebook, at its core, is a photo-sharing product. So is Instagram.

Instagram is mobile – the future. A year ago, Facebook was not.

Tumblr is not more mobile than Facebook. It is not a photo-sharing service.